An ECG signal represents changes in electrical potential produced by contractions of the heart recorded from the surface of the body. An example of an ECG signal is illustrated in FIG. 1. Each P-QRS-T complex reflects the electrical depolarization and repolarization components of the heart beat. The cardiac beat comprises three major waves identified as: the P wave representing the depolarization of the auricles, the QRS complex generated by the depolarization of both right and left ventricles, and the T-wave which represents the repolarization of the ventricles. The T wave also includes an apex, Tapex, and an endpoint, Toffset. The repolarization of the auricles is hidden in the QRS complex. The RR intervals are defined by the interval between two consecutive R peaks in successive beats. The RR interval is a direct measurement of the heart rate.
In an ECG signal, a modification of the T wave morphology can be observed when certain pharmaceutical agents are taken and also with certain types of cardiac disorders. In particular, it is known that prolongation of the QT interval in an ECG signal is clearly associated with an increased risk for ventricular arrhythmias and sudden cardiac death. Accordingly, prior systems have been developed to monitor the QT interval in ECG signals to detect potential heart problems.
Although these prior systems work, they have problems in accurately identifying the endpoint of the T wave when the endpoint of the T wave gradually approaches the baseline, when a U wave is present, or when the shape of T wave is biphasic. If the apex or the endpoint of the T wave are not accurately identified, the accuracy and robustness of repolarization analysis, i.e. analysis of the QT interval, especially with dynamic ECG signals, such as those obtained by exercise ECG testing, is compromised.